Since at least the early 1960's, liquid crystalline polymers have been used to produce high strength fibers. Well known examples of these types of fibers include aramid fibers made from highly-oriented rod-like polymers of poly(paraphenylene terephthalamide), well known as KEVLAR.RTM. aramid fibers commercially available from E.I. du Pont de Nemours and Company, Wilmington, Del. or Twaron fibers, commercially available from Akzo Nobel NV, Netherlands. These aramid fibers provide exceptional tenacity and a high tensile modulus. Breaking strengths of 2.3-3.4 gigapascals (GPa) with a modulus of 55-143 GPa are typical for these fibers. This, combined with their low specific gravity and thermal stability, has resulted in improved performance in many structural applications such as aircraft, boats, sporting goods, missiles and armor. However, a major drawback of these types of fibers has been their relatively poor flexural rigidity and compressive properties. Compressive force along the fiber axis causes buckling, which results in fiber breakdown. Fibers yield at low values of stress on the order of 400 megapascals (MPa) with the formation of kink bands.
In order to alleviate this difficulty, much effort has gone into attempts to cross-link the polymer in the filaments, but to date there has been little success. Another approach has been to coat the fiber with a sufficiently high modulus material to, in effect, "girdle" the filament and prevent buckling. Early work by McGarry et al., SAMPE Quarterly, p. 35, July 1992, demonstrated the effectiveness of this approach with vapor deposited alumina coatings. Other coatings for KEVLAR.RTM. aramid fibers with potential for improving the mechanical properties of the fibers have been explored. Recently, enhanced properties have been reported for diamond-like carbon (DLC) coatings on KEVLAR.RTM. aramid fibers.
Chemical vapor deposition (CVD) involves the reaction of gas phase species with a heated surface, resulting in a thin coating of a solid material. This process allows for greater dispersion of the chemicals to provide uniform coatings on complex shapes. One of the limitations of CVD is the high surface temperatures (&gt;600.degree. C.) required for depositions. Temperature sensitive substrates like polymeric fibers such as Kevlar.RTM. aramid fibers may be damaged under these conditions.
Thus, an object of the invention is to provide a process for the chemical vapor deposition of thin coatings on polymeric fibers.
Another object of the invention is to provide a process for the chemical vapor deposition of thin coatings of TiN on polymeric fibers and particularly onto polymeric fibers such as Kevlar.RTM. aramid fibers.
Another object of this invention is to provide an thin film coated polymeric article such as a TiN coated polyaramid fiber.